Unsaturated fatty acids and mineral oils as internal breakers for VES-gelled fluids

ABSTRACT

Fluids viscosified with viscoelastic surfactants (VESs) may have their viscosities reduced (gels broken) by the direct or indirect action of a synergistic internal breaker composition that contains at least one first internal breaker that may be a mineral oil and a second breaker that may be an unsaturated fatty acid. The internal breakers may initially be dispersed oil droplets in an internal, discontinuous phase of the fluid. This combination of different types of internal breakers break the VES-gelled aqueous fluid faster than if one of the breaker types is used alone in an equivalent total amount.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/849,780 filed Sep. 4, 2007 issued Apr. 13, 2010 as U.S. Pat. No.7,696,134 which is a continuation-in-part of U.S. patent applicationSer. No. 11/373,044 filed Mar. 10, 2006 issued Jan. 12, 2010 as U.S.Pat. No. 7,645,724, which in turn claims the benefit of U.S. ProvisionalApplication No. 60/662,336 filed Mar. 16, 2005, and is also acontinuation-in-part of U.S. patent application Ser. No. 11/517,688filed Sep. 8, 2006 issued Mar. 25, 2008 as U.S. Pat. No. 7,347,266,which in turn claims the benefit of U.S. Provisional Application No.60/717,307 filed Sep. 15, 2005.

TECHNICAL FIELD

The present invention relates to gelled aqueous treatment fluids usedduring hydrocarbon recovery operations, and more particularly relates,in one non-limiting embodiment, to methods of “breaking” or reducing theviscosity of aqueous treatment fluids containing viscoelastic surfactantgelling agents used during hydrocarbon recovery operations throughinternal breakers and particularly synergistic combinations of internalbreakers.

TECHNICAL BACKGROUND

One of the primary applications for viscosified fluids is hydraulicfracturing. Hydraulic fracturing is a method of using pump rate andhydraulic pressure to fracture or crack a subterranean formation. Oncethe crack or cracks are made, high permeability proppant, relative tothe formation permeability, is pumped into the fracture to prop open thecrack. When the applied pump rates and pressures are reduced or removedfrom the formation, the crack or fracture cannot close or healcompletely because the high permeability proppant keeps the crack open.The propped crack or fracture provides a high permeability pathconnecting the producing wellbore to a larger formation area to enhancethe production of hydrocarbons.

The development of suitable fracturing fluids is a complex art becausethe fluids must simultaneously meet a number of conditions. For example,they must be stable at high temperatures and/or high pump rates andshear rates that can cause the fluids to degrade and prematurely settleout the proppant before the fracturing operation is complete. Variousfluids have been developed, but most commercially used fracturing fluidsare aqueous-based liquids that have either been gelled or foamed. Whenthe fluids are gelled, typically a polymeric gelling agent, such as asolvatable polysaccharide, for example guar and derivatized guarpolysaccharides, is used. The thickened or gelled fluid helps keep theproppants within the fluid. Gelling can be accomplished or improved bythe use of crosslinking agents or crosslinkers that promote crosslinkingof the polymers together, thereby increasing the viscosity of the fluid.One of the more common crosslinked polymeric fluids is boratecrosslinked guar.

The recovery of fracturing fluids may be accomplished by reducing theviscosity of the fluid to a low value so that it may flow naturally fromthe formation under the influence of formation fluids. Crosslinked gelsgenerally require viscosity breakers to be injected to reduce theviscosity or “break” the gel. Enzymes, oxidizers, and acids are knownpolymer viscosity breakers.

While polymers have been used in the past as gelling agents infracturing fluids to carry or suspend solid particles as noted, suchpolymers require separate breaker compositions to be injected to reducethe viscosity. Further, such polymers tend to leave a coating on theproppant and a filter cake of dehydrated polymer on the fracture faceeven after the gelled fluid is broken. The coating and/or the filtercake may interfere with the functioning of the proppant. Studies havealso shown that “fish-eyes” and/or “microgels” present in some polymergelled carrier fluids will plug pore throats, leading to impairedleakoff and causing formation damage.

Recently it has been discovered that aqueous drilling and treatingfluids may be gelled or have their viscosity increased by the use ofnon-polymeric viscoelastic surfactants (VES). These VES materials are inmany cases advantageous over the use of polymer gelling agents in thatthey are comprised of low molecular weight surfactants rather than highmolecular polymers. The VES materials may leave less gel residue withinthe pores of oil producing formations, leave no filter cake (dehydratedpolymer) on the formation face, leave a minimal amount of residualsurfactant coating the proppant, and inherently do not create microgelsor “fish-eyes”-type polymeric masses.

However, little progress has been made toward developing internalbreaker systems for the non-polymeric VES-based gelled fluids. To thispoint, VES gelled fluids have relied only on “external” or “reservoir”conditions for viscosity reduction (breaking) and VES fluid removal(clean-up) during hydrocarbon production. Additionally, over the pastdecade it has been found that reservoir brine dilution has only a minor,if any, breaking effect of VES gel within the reservoir.

Instead, only one reservoir condition is primarily relied on for VESfluid viscosity reduction (gel breaking or thinning), and that has beenthe rearranging, disturbing, and/or disbanding of the VES worm-likemicelle structure by contacting the hydrocarbons within the reservoir,more specifically contacting and mixing with crude oil and condensatehydrocarbons. SPE 30114 describes how reservoir hydrocarbons reduce theviscosity of VES-gelled fluids. SPE 31114 notes that when a VES-gelledfluid contacts crude or condensate reservoir hydrocarbons, theVES-gelled fluid will break, i.e. lose viscosity. SPE 60322 describeshow oil or gas reservoir hydrocarbons alter the worm-like micelles of aVES-gelled fluid into spherical micelle structures which results inwater-like fluid viscosity. SPE 82245 explains that contact of aVES-gelled fluid system with hydrocarbons causes the fluid to lose itsviscosity.

However, in many gas wells and in cases of excessive displacement ofcrude oil hydrocarbons from the reservoir pores during a VES geltreatment, results have shown many instances where VES fluid in portionsof the reservoir are not broken or are incompletely broken resulting inresidual formation damage (hydrocarbon production impairment). In suchcases post-treatment clean-up fluids composed of either aromatichydrocarbons, alcohols, surfactants, mutual solvents, and/or other VESbreaking additives have been pumped within the VES treated reservoir inorder to try and break the VES fluid for removal. However, placement ofclean-up fluids is problematic and normally only some sections of thereservoir interval are cleaned up, leaving the remaining sections withunbroken or poorly broken VES gelled fluid that impairs hydrocarbonproduction. Because of this phenomenon and other occasions wherereliance on external factors or mechanisms has failed to clean up theVES fluid from the reservoir during hydrocarbon production, or in caseswhere the external conditions are slow acting (instances where VESbreaking and clean-up takes a long time, such as several days up topossibly months) to break and then produce the VES treatment fluid fromthe reservoir, and where post-treatment clean-up fluids (i.e. use ofexternal VES breaking solutions) are inadequate in removing unbroken orpoorly broken VES fluid from all sections of the hydrocarbon bearingportion of the reservoir, there has been an increasing and importantindustry need for VES fluids to have internal breakers. Desirableinternal breakers that should be developed include breaker systems thatuse products that are incorporated within the VES-gelled fluid that areactivated by downhole temperature that will allow a controlled rate ofgel viscosity reduction over a rather short period of time of 1 to 8hours or so, similar to gel break times common for conventionalcrosslinked polymeric fluid systems.

A challenge has been that VES-gelled fluids are not comprised ofpolysaccharide polymers that are easily degraded by use of enzymes oroxidizers, but are comprised of low molecular weight surfactants thatassociate and form viscous rod- or worm-shaped micelle structures.Conventional enzymes and oxidizers have not been found to act anddegrade the surfactant molecules or the viscous micelle structures theyform. It is still desirable, however, to provide some mechanism thatrelies on and uses internal phase breaker products that will help assurecomplete viscosity break of VES-gelled fluids.

It would be desirable if a viscosity breaking system could be devised tobreak the viscosity of fracturing and other well completion fluidsgelled with and composed of viscoelastic surfactants, particularly breakthe viscosity completely and relatively quickly. It would beparticularly desirable if the breakers used could be used in relativelysmall amounts to save on material costs.

SUMMARY

There is provided, in one non-limiting form, a method for breaking theviscosity of aqueous fluids gelled with a viscoelastic surfactant (VES)that involves adding to an aqueous fluid at least one VES in an amounteffective to increase the viscosity thereof. The method further includesadding to the aqueous fluid before, during or after the VES at least onefirst internal breaker in an amount effective to reduce the viscosity ofthe gelled aqueous fluid. The first internal breaker is a mineral oil.Further, the method involves adding to the aqueous fluid before, duringor after the VES at least one second internal breaker in an amounteffective to reduce the viscosity of the gelled aqueous fluid. Thesecond internal breaker may be one or more unsaturated fatty acid,including a polyenoic acid and/or a monoenoic acid.

In another non-restrictive embodiment, there is provided an aqueousfluid that contains at least one viscoelastic surfactant (VES) in anamount effective to increase the viscosity of the aqueous fluid. Thefluid also contains at least one first internal breaker in an amounteffective to reduce the viscosity of the gelled aqueous fluid, where thefirst internal breaker is again a mineral oil. There is also present inthe fluid at least one second internal breaker in an amount effective toreduce the viscosity of the gelled aqueous fluid, where the secondinternal breaker is an unsaturated fatty acid, which may be a polyenoicacid and/or a monoenoic acid.

The methods and compositions that will be described in further detailbelow allow the internal breakers to work in relatively moderate to highsalinity brine mix waters and/or low temperatures. Moderate to highsalinities in the aqueous fluids have the effect of slowing the rate andcompleteness of final break when internal breakers are used in aqueousfluids gelled with a VES. It has been discovered that using twodifferent types of internal breakers overcomes the rate-slowing effectthat salinity has on the internal breakers, including at lower andhigher temperatures. By using both of these internal breakers VES-gelledfluids having high salinity may be easily broken. This permits the useof a lower amount of one or the other or both of the internal breakertypes. This may have a number of benefits including, but not necessarilylimited to, lowering the overall cost to break a VES-gelled fluid byrequiring less of one or both internal breakers when the other breakeris present, requiring less of one or both internal breakers to achieve acomplete viscosity break, and/or allowing complete VES-gelled fluidviscosity breaks to be achieved more quickly when using both of theseinternal breakers, as compared to an identical method or compositionwhere only one of the breaker types is used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the viscosity breaking results as a functionof time using five different formulations of a brine that contained 9%KCl, 6% WG-3L VES, with 20.0 pptg (2.4 kg/m³) FLC-40 fluid loss controlagent, with and without a polyenoic acid internal breaker, without andwith varying amounts of a transition metal ion source rate controllerand with and without a mineral oil internal breaker, demonstrating asynergistic result using both the polyenoic acid internal breaker andthe mineral oil internal breaker, as compared when each is usedseparately; and

FIG. 2 is a graph showing the viscosity breaking results as a functionof time using three different formulations of a brine having 14.0 ppg(1.7 kg/liter) CaBr₂, 5.0% WG-3L VES, and 15 pptg (1.8 kg/m³) FLC-40viscosity stabilizer and fluid loss control agent, first with only apolyenoic acid internal breaker, then with only a mineral oil internalbreaker, and then using both, proving a synergistic result using bothtypes of internal breakers.

DETAILED DESCRIPTION

Synergistic results have been surprisingly discovered when both anunsaturated fatty acid (UFA) (e.g. a polyenoic acid) and a mineral oilare used together as internal breakers to reduce the viscosity ofVES-gelled aqueous fluids. Unexpectedly, the use of both types ofbreakers together more easily and more efficiently breaks VES-gelledaqueous fluids. As the data discussed below will show, faster VES-gelbreaking may occur than would be expected by simply combining what eachindividual VES internal breaker contributes alone. The discovery appearsto be a very localized weakening of the VES micelles, analogous to a“pinhole” effect, so that the micelles become much more susceptible toone or both of the VES breakers. This “pinhole effect” may be considereda weakening, e.g. a reduction in the thermodynamic energy that existsbetween the VES head groups with themselves, water and counter-ionspresent (such as the salts in the brine) at a few locations on the VESmicelle structure, in one non-limiting explanation. This suspectedweakening appears to allow one or the other or both VES internalbreakers that normally are thermodynamically unable to penetrate andcollapse the micelle to somehow cause weakened locations where one orboth breakers may enter the micelle and the process proceedscontrollably until complete or near-complete breaking of the VES-geloccurs.

The importance of this discovery will allow less amount of one or theother or both internal breakers to be required to perform a given rateof viscosity reduction or given completeness of reduction or both, or tospeed up and to more completely break the VES-gelled fluid. The use ofthese two types of breakers will overcome the negative effects that mixwater salinity normally has on internal breakers, and will allow fasterand more complete breaks to occur at lower and higher temperatures thanwhen the breakers are used individually. That is, this discovery expandsand enhances these internal breaking technologies.

More specifically, the simultaneous use of both types of internalbreakers may allow the internal breakers to work in a wide range ofsalinity brine mix waters, in non-limiting examples from about 1% toabout 24% by weight (bw) KCl; about 1% to about 44% bw NaBr, about 1% toabout 37% bw CaCl₂ brine; about 1% to about 63% bw CaBr₂ brine; etc. Theuse of both of these two internal breaker types may overcome therate-slowing effect salinity has on internal breakers at alltemperatures, including very low temperatures, and thus mineral oil-typeinternal breakers and unsaturated fatty acid-type breakers may now beused together at ambient temperatures, such as about 80° F. (about 27°C.), to break VES fluids that have high salinity. By using both types ofinternal breakers, lower amounts of the polyenoic acid and/or themineral oil breakers may be used, as compared with an identical fluidwhere only one internal breaker type is used. This will give advantagesthat include, but are not necessarily limited to, lowering the overallcost to break a VES-gelled aqueous fluid by requiring less internalbreaker when the other breaker type is present, lowering the amount ofinternal breaker to achieve complete VES viscosity break, allowing acomplete VES fluid viscosity break to be achieved more quickly than whenusing polyenoic or mineral oil breakers alone and combinations of these.

The viscoelastic surfactants are believed to impart viscosity to anaqueous fluid by the molecules organizing themselves into micelles.Spherical micelles do not give increased viscosity, however, when themicelles have an elongated configuration, for instance are “rod-shaped”or “worm-shaped”, they become entangled with one another therebyincreasing the viscosity of the fluid.

In one non-limiting embodiment gel-breaking products, such as theinternal breakers herein, work by rearrangement of the VES micelle fromrod-shaped or worm-shaped elongated structures to spherical structures:that is, the collapse or rearrangement of the viscous elongated micellestructures to nonviscous, more spherical micelle structures.Disaggregating may be understood in one non-limiting embodiment when themicelles are not closely associated physically, that is are no longeraggregated or physically interacted together thereby resulting inreduced fluid viscosity, as contrasted with rearrangement which may beunderstood as a different physical and chemical arrangement oraggregation of the multi-surfactant micelles that has reduced viscosity.However, the inventors do not necessarily want to be limited to anyparticular mechanism or explanation.

Elongated VES structures may sometimes be referred to as “living”because there is a continuous exchange of VES-type surfactants leavingthe VES structures to the aqueous solution and other surfactants leavingthe aqueous solution and entering or re-entering the VES micellestructures. It is suspected, in one non-restrictive explanation, thatthe unsaturated fatty acids over time and under specific conditionsbecome auto-oxidized and dispersed in the VES elongated micelles andthereby allow a small opening to occur in the VES micelles that permitsthe mineral oil internal breakers to enter and/or further destabilizethe VES micellar structure, somewhat analogous to a chemical “pinhole”or localized position that opens up or facilitates disruption of themicelles by the internal breakers. However, the inventors do not wish tobe limited to any particular explanation or mechanism. It appears thatuse of both types of internal breakers lowers the free energy requiredto penetrate the VES micelle head groups, and allows or permits, orappears to transport carry the mineral oil molecules into the VESmicelle. Thus, the ability of the mineral oil internal breaker to breakthe VES structures more quickly and completely is enhanced. Again, itseems to be that the use of UFA internal breakers distributes andcreates “pinholes” or localized weakened VES head group arrangements orconfigurations in the VES structure that allows the mineral oil internalbreaking agents to associate and further weaken the “pinhole” locally.In any event, the overall result is that the mineral oil internalbreaker works more quickly in degrading the VES fluid viscosity when theunsaturated fatty acid is present in the fluid. That is, the use of bothtypes of breakers at least partially overcome the negative influences ofmix water salinity, using them both together allow the internal breakersto work more effectively and/or more quickly at lower concentrationsand/or higher mix water salinity. The result is that internal breakersmay be used in situations or environments when they normally are lesseffective or are effective only at very high concentrations. Thisdiscovery expands the domain and use of internal breakers, improvestheir effectiveness, and reduces the cost of using them.

The internal breaker components herein may be added safely and easily tothe gel after batch mixing of a VES-gel treatment, or added on-the-flyafter continuous mixing of a VES-gel treatment using a liquid additivemetering system in one non-limiting embodiment, or the components may beused separately, if needed, as an external breaker solution to removeVES gelled fluids already placed downhole. The mineral oils (beinginherently hydrophobic) and/or the mono- and/or polyenoic acids are notsolubilized in the brine, but rather interact with the VES surfactantand/or remain as oil droplets to be dispersed and form an emulsion (oilin water type emulsion) and thus there is an oil-stabilized emulsiondispersed in the “internal phase” as a “discontinuous phase” or as an“oil-soluble” internal phase of the brine medium/VES fluid which is the“outer phase” or “continuous phase”. It appears in some cases that themineral oils, e.g., or unsaturated fatty acids (UFAs) are evenlydispersed and are incorporated within the viscous rod- or worm-likeshape micelles, however, in other cases, particularly for mineral oilsand other saturated hydrocarbons, the oil breaker component can remainas droplets outside of the VES micelles, or as a combination of bothmicellular locations to various degrees. Rheometer tests have shown,that the incorporation of the UFAs into (i.e. dispersed and/ordistributed throughout, within or a part of) the VES micelles does notdisturb the elongated structure and viscosity yield of the VES micellesat the levels or amounts of UFAs needed to obtain a complete VES gelviscosity break. This is remarkably unique since oils are considered toreadily break the viscosity of VES fluids upon contact. The UFAs inparticular, have high compatibility with VES fluids until they undergonatural or induced auto-oxidation, whereby the auto-oxidationby-products have been found to readily disturb VES micelles structuresand fluid viscosity.

It is surprising and unexpected that mineral oils may serve as internalbreakers. This is surprising because the literature teaches that“contact” of a VES-gelled fluid with hydrocarbons, such as those of theformation in a non-limiting example, essentially instantaneously reducesthe viscosity of the gel or “breaks” the fluid. By “essentiallyinstantaneously” is meant less than one-half hour. The rate of viscositybreak for a given reservoir temperature by the methods described hereinis controlled by type and amount of salts within the mix water (i.e.seawater, KCl, NaBr, CaCl₂, CaBr₂, NH₄Cl and the like), presence of aVES gel stabilizer (i.e. MgO, ZnO and the like), presence of aco-surfactant (i.e. sodium dodecyl sulfate, sodium dodecyl benzenesulfonate, potassium laurate, potassium oleate, sodium lauryl phosphate,and the like), VES type (i.e. amine oxide, quaternary ammonium salt, andthe like), VES loading, the amount of mineral oil used, the distillationrange of the mineral oil, its kinematic viscosity, the presence ofcomponents such as aromatic hydrocarbons, and now, of course, thepresence of a polyenoic unsaturated fatty acid internal breaker.

It is important in most non-limiting embodiments herein to add the lowermolecular weight or low viscosity mineral oil products after the aqueousfluid is substantially gelled. Addition of the lower molecular weightmineral oil prior to substantial gelling tends to prevent the gelling orviscosity increase to occur. By “substantially gelled” is meant that atleast 90% of the viscosity increase has been achieved before theinternal breaker (e.g. mineral oil) is added. Of course, it isacceptable to add the lower molecular weight mineral oil after the gelhas completely formed. However, in another non-limiting embodiment, whenusing the higher molecular weight or higher viscosity mineral oils theorder of addition is not important, that is, these type of mineral oilsmay be added prior to, during, or after the VES product is added to theaqueous fluid and gelled.

Mineral oil (also known as liquid petrolatum) is a by-product in thedistillation of petroleum to produce gasoline. It is a chemically inerttransparent colorless oil composed mainly of linear, branched, andcyclic alkanes (paraffins) of various molecular weights, related towhite petrolatum. Mineral oil is produced in very large quantities, andis thus relatively inexpensive. Mineral oil products are typicallyhighly refined, through distillation, hydrogenation, hydrotreating, andother refining processes, to have improved properties, and the type andamount of refining varies from product to product. Highly refinedmineral oil is commonly used as a lubricant and a laxative, and withadded fragrance is marketed as “baby oil” in the U.S. Most mineral oilproducts are very inert and non-toxic, and are commonly used as babyoils and within face, body and hand lotions in the cosmetics industry.Other names for mineral oil include, but are not necessarily limited to,paraffin oil, paraffinic oil, lubricating oil, white mineral oil, andwhite oil.

In one non-limiting embodiment the mineral oil may be at least 99 wt %paraffinic. Because of the relatively low content of aromatic compounds,mineral oil has a better environmental profile than other oils. Ingeneral, the more refined and less aromatic the mineral oil, the better.In another non-restrictive version, the mineral oil may have adistillation temperature range from about 160 to about 550° C.,alternatively have a lower limit of about 200° C. and independently anupper limit of about 480° C.; and a kinematic viscosity at 40° C. fromabout 1 to about 250 cSt, alternatively a lower limit of about 1.2independently to an upper limit of about 125 cSt. Specific examples ofsuitable mineral oils include, but are not necessarily limited to,BENOL®, CARNATION®, KAYDOL®, SEMTOL®, HYDROBRITE® and the like mineraloils available from Crompton Corporation, ESCAID®, EXXSOL® ISOPAR® andthe like mineral oils available from ExxonMobil Chemical, and similarproducts from other mineral oil manufacturers. A few non-limitingexamples are specified in Table I. The ESCAID 110® and CONOCO LVT-200®mineral oils have been well known components of oil-based drilling mudsand the oil industry has considerable experience with these products,thus making them an attractive choice. The white mineral oils fromCrompton Corporation with their high purity and high volume use withinother industries are also an attractive choice.

TABLE I PROPERTIES OF VARIOUS MINERAL OILS ESCAID EXXSOL HYDROBRITEHYDROBRITE 110 D110 ISOPARV BENOL 200 1000 Properties Specific Gravity0.790-0.810 0.780-0.830 0.810-0.830 0.839-0.855 0.845-0.885 0.860-0.885Viscosity @ 1.3-1.9 — — 18.0-20.0 39.5-46.0 180.0-240.0 40° C. FlashPoint (° C.)   77.0 105 118 186 — 288 Pour Point (° C.) — — — −21.0 −9.0−6.0 Distillation Range IBP (° C.) 200 237 263 — — — Max DP (° C.) 248277 329 — — — GC Distillation — — — — >380 >407 5% (° C.) Molecular Wt.— — — — — >480 Aromatic <0.5% <1.0% <0.5% — — — Content Note: ESCAID,EXXSOL and ISOPAR are trademarks of ExxonMobil Corporation. BENOL andHYDROBRITE are trademarks of Crompton Corporation.

It has been discovered in breaking VES-gelled fluids prepared inmonovalent brines (such as 3% KCl brine) that at temperatures belowabout 180° F. (82° C.) ESCAID® 110 mineral oil works well in breakingVES-gelled fluids, and that at or above about 140° F. (60° C.)HYDROBRITE® 200 mineral oil works well. The use of mineral oils hereinis safe, simple and economical. In some cases for reservoir temperaturesbetween about 120° to about 240° F. (about 49° to about 116° C.) aselect ratio of two or more mineral oil products, such as 50 wt %ESCAID® 110 mineral oil to 50 wt % HYDROBRITE® 200 mineral oil may beused to achieve controlled, fast and complete break of a VES-gelledfluid. The use of the polyenoic acids herein expands the ranges of brinesalt concentrations for which these mineral oils are useful.

It has also been discovered that the type and amount of salt within themix water used to prepare the VES fluid (such as 3 wt % KCl, 21 wt %CaCl₂, use of natural seawater, and so on) and/or the presence of a VESgel stabilizer (such as VES-STA 1 gel stabilizer available from BakerOil Tools) may affect the activity of a mineral oil in breaking a VESfluid at a given temperature. For example, ESCAID® 110 mineral oil at5.0 gptg will readily break the 3 wt % KCL based VES fluid at 100° F.(38° C.) over a 5 hour period, and ESCAID® 110 mineral oil may stillhave utility as a breaker for a 10.0 ppg CaCl₂ (21 wt % CaCl₂) based VESfluid at 250° F. (121° C.).

Other refinery distillates may potentially be used in addition to oralternatively to the mineral oils described herein, as may behydrocarbon condensation products. Additionally, synthetic mineral oils,such as hydrogenated polyalphaolefins, and other synthetically derivedsaturated hydrocarbons may be of utility to practice the methods herein.More information about the use of mineral oils, hydrogenatedpolyalphaolefin oils, and saturated fatty acids as internal breakers maybe found in U.S. Patent Application Publication 2007/0056737 A1published Mar. 15, 2007, incorporated by reference herein in itsentirety. In another non-limiting embodiment, natural unsaturatedhydrocarbons such as terpenes (e.g. pinene, d-limonene, etc.), saturatedfatty acids (e.g. lauric acid, palmitic acid, stearic acid, etc. fromplant, fish and/or animal origins) and the like may possibly be usedtogether with or alternatively to the mineral oils herein.

With respect to the unsaturated fatty acid internal breakers such asmonoenoic acids and polyenoic acids, as internal breakers, in onenon-limiting embodiment these may be specific oils that contain arelatively high amount of either monoenoic or polyenoic acids or both.There are many books and other literature sources that list the multipletypes and amounts of fatty acids compositions of oils and fats availablefrom plant, fish, animal, and the like. A polyenoic acid is definedherein as any of various fatty acids having more than one double bond(allyl group) in the carbon chain, e.g. linoleic acid. Correspondingly,a monoenoic acid is a fatty acid having only one double bond (allylgroup). The terms unsaturated fatty acid (UFA) or unsaturated fattyacids (UFAs) are defined herein as oils or fats containing one or theother or both monoenoic and polyenoic fatty acids. Other suitablepolyenoic acids include, but are not necessarily limited to omega-3fatty acids, and omega-6 fatty acids, stearidonic acid, eleostearicacid, eicosadienoic acid, eicosatrienoic acid, arachidonic acid oreicosatetraenoic acid (ETA), eicosapentaenoic acid (EPA),docosapentaenoic acid, docosahexaenoic acid (DHA), cis-linoleic acid,cis-linolenic acid, gamma-linolenic acid, conjugated polyenes, andmixtures thereof. Other suitable monoenoic acids include, but are notnecessarily limited to obtusilic acid, caproleic acid, lauroleic acid,linderic acid, myristoleic acid, physeteric acid, tsuzuic acid,palmitoleic acid, petroselinic acid, oleic acid, vaccenic acid, gadoleicacid, gondoic acid, cetoleic acid, nervonic acid, erucic acid, elaidicacid, t-vaccenic acid, and mixtures thereof.

Oils relatively high in monoenoic and polyenoic acids include, but arenot necessarily limited to, flax (linseed) oil, soybean oil, olive oil,canola (rapeseed) oil, chia seed oil, corn oil, cottonseed oil, eveningprimrose oil, grape seed oil, pumpkin seed oil, safflower oil, sunfloweroil, walnut oil, peanut oil, various fish oils, mammal oils, and animaloils or fats and the like.

Any of these oils or fats may be partially hydrogenated, or may containoriginal or additional preservatives, such as tocopherols, and the like.Additionally any one or more of these oils may be “aged” before use toadjust the product's auto-oxidation activity, along with any one or morereagent or technical grade fatty acids. Allowing a specific fatty acidor UFA oil to “age” allows auto-oxidation to initiate and progressdependant on the amount of time, environmental conditions (temperature,exposure to atmosphere, etc.), presence of other compounds (tocopherols,metal ions, etc), and the like.

It appears that the more double-bonded carbons on the fatty acid carbonchain the more active that fatty acid will be in auto-oxidation, thatis, these materials auto-oxidize easier and more quickly. This seems tobe a general rule, although other components in the oil may alter thisrule. Table II lists the relative rates of oxidation of common fattyacids, from the “Autooxidation” section within “Chemical Reactions ofOil, Fat, and Based Products”, Department of Engineering, InstitutoSuperior T'echnico, Lisbon, Portugal, October 1997.

TABLE II RELATIVE OXIDATION RATES OF SOME COMMON FATTY ACIDS Totalamount of Number of double Relative rate of Fatty acid carbon atomscarbon bonds oxidation Stearic 18 0 1 Oleic 18 1 100 Linoleic 18 2 1200Linolenic 18 3 2500

Unsaturated fatty acids have been found to break down by“auto-oxidation” into a gamut of VES-breaking products or compositions.Each oil with various monoenoic and polyenoic acids uniquely shows thebreakdown of the VES surfactant micelle structure by the presence ofthese auto-oxidation generated byproducts. Auto-oxidation is also knownas autooxidation and lipid peroxidation which includes the oxidation ofunsaturated fatty acids. Auto-oxidation in this context may also includea chain reaction—multiple steps and chemical species occur in theoxidative breakdown. Various hydroperoxides may be formed in theseauto-oxidations, and end products typically include, but are notnecessarily limited to, carbonyl compounds (various aldehydes andketones), alcohols, acids, and “hydrocarbons” of various types, e.g.alkanes, saturated fatty acids and the like, and mixtures thereof. Avariety of technical books and papers list many of the numerous productsgenerated by auto-oxidation (autooxidation) of unsaturated fatty acids.

Fatty acids may also decompose in a water medium and alkaline conditionby hydrolysis.

It may be possible that other olefins (e.g. allyl group compounds) maybe investigated and employed in the same manner that unsaturated fattyacids have been found to work toward breaking VES-gelled fluids. It alsomay be possible that mechanisms other than oxidation or hydrolysis maybe functioning in generating VES breaking compounds from olefins andolefin derivatives, although the inventors do not want the methods andcompositions herein to be limited by any supposed theory. Moreinformation about the use of mono- and polyenoic acids (UFAs) asinternal breakers may be found in U.S. Patent Application Publication2006/0211776 A1 published Sep. 21, 2006, incorporated by referenceherein in its entirety.

The breaking or viscosity reduction may be triggered or initiated byheat for both types of internal breakers. The mineral oils and relatedinternal breakers will slowly, upon heating, break or reduce theviscosity of the VES gel with the addition of or in the absence of anyother viscosity reducing agent. The amount of mineral oil needed tobreak a VES-gelled fluid appears to be somewhat temperature dependent,with less needed as the fluid temperature increases and fluid salinitydecreases, unless one of the polyenoic internal breakers describedherein is included. The kinematic viscosity, molecular weightdistribution, and amount of impurities (such as aromatics, olefins, andthe like) also appear to influence the rate in which a mineral oil willbreak a VES-gelled fluid at a given temperature. Once a fluid iscompletely broken a degree of viscosity reheal may occur but in mostcases no rehealing is expected. An effective amount of mineral oilranges from about 50 to about 25,000 ppm based on the total fluid, inanother non-limiting embodiment from a lower limit of about 500.Independently the upper limit of the range may be about 10,000 ppm basedon the total fluid. The necessary proportions or amounts are expected tobe lower in the presence of the polyenoic acid internal breaker. (Itwill be appreciated that units of gallon per thousand gallons (gptg) arereadily converted to SI units of the same value as, e.g. liters perthousand liters.)

The mono- and polyenoic acids will slowly to fairly rapidly, uponheating or subjecting the acids to a temperature, auto-oxidize into theVES gel breaking compounds with the addition of or in the absence of anyother agent. The amount of altered or oxidized unsaturated fatty acidneeded to break a VES-gelled fluid appears to be salinity-, VESconcentration- and temperature-dependent, with typically more needed assalinity increases, as the VES concentration increases, and less neededas fluid temperature increases. Of course, as previously discussed, thepresence of a mineral oil internal breaker will facilitate the reductionof the VES-caused viscosity and lower the temperatures necessary forbreaking to occur. Once a fluid is completely broken a degree ofviscosity reheal may occur but in most cases no reheal in viscosity willoccur and no phase separation of the VES occurs upon fluid cool down,that is when the test fluid is left at test temperature for a sufficientamount of time for complete to near-complete auto-oxidation of themonoenoic and/or polyenoic acids to occur. In one non-limitingembodiment, at least one second internal breaker, e.g. polyenoic acid,is present an amount from about 100 to about 20,000 ppm based on thetotal fluid; alternatively in an amount from about 600 ppm,independently up to about 12,000 ppm.

Controlled viscosity reduction rates using the internal breakers may beachieved at a temperature of from about 70° F. to about 300° F. (about21 to about 149° C.), and alternatively at a temperature of from about100° F. independently to an upper end of the range of about 280° F.(about 38 to about 138° C.). The temperature range may be shifted lowerwith the use of the both internal breaker types used herein. It has alsobeen discovered that VES-gelled aqueous fluids containing the smallamounts of mineral oils described herein are relatively shear stable andcan tolerate some shear before viscosity reduction occurs. In onenon-limiting embodiment, the fluid designer would craft the fluid systemin such a way that the VES gel would break at or near the expectedformation temperature after fracturing was accomplished.

Fluid design may be based primarily on formation temperature, i.e. thetemperature the fluid will be heated to naturally in the formation oncethe treatment is over. Further, fluid design may be based on theexpected cool down of the fluid during a treatment. In many cases thefracturing fluid may only experience actual reservoir temperature for 5%to 25% of the job time, and close to 50% of the fluid is never exposedto the original reservoir temperature because of the cool down of thereservoir by the initial fracturing fluid placed into the reservoir. Itis because a portion of the fracturing fluid (or other fluid) will notsee or be exposed to the original reservoir temperature that a coolertemperature is selected that will represent what the fluid will probablysee or contact, and thus laboratory break tests are sometimes run at acooler temperature. There would generally be no additional temperaturethe VES fluid would see other than original reservoir temperature. Thefluid design may also be influenced by the salinity of the mix water,particularly the presence of divalent ion type mix waters, with thehigher the salinity the greater the amount of combined internal breakersrequired.

The use of the disclosed dual internal breaker system is ideal forcontrolling viscosity reduction of VES-based fracturing fluids. Thebreaking system may also be used for breaking gravel pack fluids,acidizing or near-wellbore clean-up diverter fluids, and losscirculation pill fluids composed of VES. The breaker system mayadditionally work for foamed fluid applications (hydraulic fracturing,acidizing, and the like), where N₂ or CO₂ gas is used for the gas phase.The VES breaking methods and compositions herein are a significantimprovement in that they give breaking rates for VES-based fluids thatthe industry is accustomed to with conventional polymer based fracturingfluids, such as borate crosslinked guar. Potentially more importantly,in another non-limiting example, the use of the dual internal breakersystems should help assure and improve hydrocarbon production comparedto prior art that uses only external mechanisms to break the VES fluidfor effective and complete VES fluid clean-up after a treatment.

In one non-limiting embodiment, the compositions herein will directlydegrade the gel created by a VES in an aqueous fluid, and alternativelywill reduce the viscosity of the gelled aqueous fluid either directly,or by disaggregation or rearrangement of the VES micellar structure(e.g. collapsing or disturbing the structure). However, the inventors donecessarily not want to be limited to any particular mechanism.

In another non-limiting embodiment, the composition may be modified toslow down or to increase the auto-oxidation of the unsaturated fattyacids. Addition of compounds that influence the rate of auto-oxidationis an important option for the methods and fluids herein, in particularfor the lower temperatures to increase the auto-oxidation rate and athigher temperatures to slow down the auto-oxidation rate. Rate controlcompounds that may be used for slowing down rate of monoenoic andpolyenoic acids may be antioxidants such as, but not limited totocopherol (vitamin E), ascorbic acid (vitamin C), butylatedhydroxytoluene (BHT) and other like preservatives, chelants (such ascitric acid, phosphates, and EDTA), amino acids, proteins, sugaralcohols (e.g. mannitol, xylitol, lactitol, and sorbitol), salts (suchas NaCl, MgCl₂, CaCl₂, NaBr and CaBr₂), and the like. Rate controlcompounds that may increase the rate of auto-oxidation may be oxidantsor pro-oxidants such as, but not limited to persulfate, percarbonate,perbromate, iron, copper, manganese and other transition metals, and thelike. It should be noted that there are numerous compounds that may beof utility for regulating the rate of auto-oxidation. The proportion ofrate control compounds that may be advantageously used may range from alower limit of about 0.00001% by weight to an upper limit of about 62%by weight, based on the total weight of fluid, and alternatively from alower limit of 0.0001% by weight and/or to an upper limit of about 45%by weight. It may be noted that rate controllers used toward the lowerlimit may be items such as metal ions and rate controllers employedtoward the upper limit may be items such as monovalent and/or divalentsalts. Chelation of the metal ions tends to slow the rate ofauto-oxidation as compared with non-chelated forms of the same metalions. In one non-limiting understanding, the use of metal ions (whetheror not chelated) may be understood as “catalyzing” the auto-oxidation ofthe UFA.

It is sometimes difficult to specify with accuracy in advance the amountof the various internal breaking components that should be added to aparticular aqueous fluid gelled with viscoelastic surfactants tosufficiently or fully break the gel, in general. For instance, a numberof factors affect this proportion, including but not necessarily limitedto, the particular VES used to gel the fluid; the particular internalbreaker used; the particular oil used as a carrier in the case of theunsaturated fatty acids; whether or not a rate-controlling agent is usedand what kind; the temperature of the fluid; the downhole pressure ofthe fluid, the starting pH of the fluid; and the complex interaction ofthese various factors.

Nevertheless, in order to give an approximate feel for the proportionsof the various breaking components to be used in the methods herein,approximate ranges will be provided. Effective amounts of mineral oilswere given previously. In an alternative, non-limiting embodiment theamount of hydrogenated polyalphaolefin oil and saturated fatty acid thatmay be effective in the methods and compositions herein may range fromabout 25 to about 25,000 ppm, based on the total amount of the fluid. Inanother non-restrictive version herein, the amount of hydrogenatedpolyalphaolefin oil and saturated fatty acid may range from a lower endof about 400 independently to an upper end of about 16,000 ppm. Theamount of unsaturated fatty acid that may be effective in the methodsand compositions may range from about 100 to about 20,000 ppm, based onthe total amount of the fluid. In another non-restrictive version, theamount of unsaturated fatty acid may range from a lower limit of about500 and/or to an upper limit of about 8,000 ppm.

Any suitable mixing apparatus may be used for the methods and fluidsherein. In the case of batch mixing, the VES and the aqueous fluid areblended for a period of time sufficient to form a gelled or viscosifiedsolution. The internal breaker, particularly the mineral oil, mayadvantageously be added after the fluid is formulated or at least afterthe fluid is substantially gelled. The VES that is useful herein may beany of the VES systems that are familiar to those in the well serviceindustry, and may include, but are not limited to, amines, amine salts,quaternary ammonium salts, amidoamine oxides, amine oxides, mixturesthereof and the like. Suitable amines, amine salts, quaternary ammoniumsalts, amidoamine oxides, and other surfactants are described in U.S.Pat. Nos. 5,964,295; 5,979,555; and 6,239,183, incorporated herein byreference in their entirety.

Viscoelastic surfactants improve the fracturing (frac) fluid performancethrough the use of a polymer-free system. These systems, compared topolymeric based fluids, can offer improved viscosity breaking, highersand trans-port capability, are in many cases more easily recoveredafter treatment than polymers, and are relatively non-damaging to thereservoir with appropriate contact with sufficient quantity of reservoirhydrocarbons, such as crude oil and condensate. The systems are alsomore easily mixed “on the fly” in field operations and do not requirenumerous co-additives in the fluid system, as do some prior systems.

The viscoelastic surfactants suitable for use herein include, but arenot necessarily limited to, non-ionic, cationic, amphoteric, andzwitterionic surfactants. Specific examples of zwitterionic/amphotericsurfactants include, but are not necessarily limited to, dihydroxylalkyl glycinate, alkyl ampho acetate or propionate, alkyl betaine, alkylamidopropyl betaine and alkylimino mono- or di-propionates derived fromcertain waxes, fats and oils. Quaternary amine surfactants are typicallycationic, and the betaines are typically zwitterionic. The thickeningagent may be used in conjunction with an inorganic water-soluble salt ororganic additive such as phthalic acid, salicylic acid or their salts.

Some non-ionic fluids are inherently less damaging to the producingformations than cationic fluid types, and are more efficacious per poundthan anionic gelling agents. Amine oxide viscoelastic surfactants havethe potential to offer more gelling power per pound, making it lessexpensive than other fluids of this type.

The amine oxide gelling agents RN⁺(R′)₂O⁻ may have the followingstructure (I):

where R is an alkyl or alkylamido group averaging from about 8 to 24carbon atoms and R′ are independently alkyl groups averaging from about1 to 6 carbon atoms. In one non-limiting embodiment, R is an alkyl oralkylamido group averaging from about 8 to 16 carbon atoms and R′ areindependently alkyl groups averaging from about 2 to 3 carbon atoms. Inan alternate, non-restrictive embodiment, the amidoamine oxide gellingagent is Akzo Nobel's Aromox® APA-T formulation, which should beunderstood as a dipropylamine oxide since both R′ groups are propyl.

Materials sold under U.S. Pat. No. 5,964,295 include CLEAR-FRAC™, whichmay also comprise greater than 10% of a glycol. One preferred VES is anamine oxide. As noted, a particularly preferred amine oxide is APA-T,sold by Baker Oil Tools as SURFRAQ™ VES. SURFRAQ is a VES liquid productthat is 50% APA-T and greater than 40% propylene glycol. Theseviscoelastic surfactants are capable of gelling aqueous solutions toform a gelled base fluid. The internal breaker additives herein may beused to prepare a VES system sold by Baker Oil Tools as DIAMOND FRAQ™fracturing fluid system. DIAMOND FRAQ™ with its assured breakingtechnology overcomes reliance on external reservoir conditions in orderto break, as compared with products such as CLEARFRAC™ fracturing fluidsystem.

The methods and compositions herein also cover commonly known materialsas AROMOX® APA-T manufactured by Akzo Nobel and other known viscoelasticsurfactant gelling agents common to stimulation treatment ofsubterranean formations.

The amount of VES included in the fracturing fluid depends on at leasttwo factors. One involves generating enough viscosity to control therate of fluid leak off into the pores of the fracture, and the secondinvolves creating a viscosity high enough to keep the proppant particlessuspended therein during the fluid injecting step, in the non-limitingcase of a fracturing fluid. Thus, depending on the application, the VESis added to the aqueous fluid in concentrations ranging from about 0.5to 25% by volume, alternatively up to about 12 vol % of the totalaqueous fluid (from about 5 to 120 gptg). In another non-limitingembodiment, the range for the present formulations is from about 1.0 toabout 6.0% by volume VES product. In an alternate, non-restrictive form,the amount of VES ranges from a lower limit of about 2 independently toan upper limit of about 10 volume %.

It is expected that the breaking compositions herein be used to reducethe viscosity of a VES-gelled aqueous fluid regardless of how theVES-gelled fluid is ultimately utilized. For instance, the viscositybreaking compositions could be used in all VES applications including,but not limited to, VES-gelled friction reducers, VES viscosifiers forloss circulation pills, fracturing fluids (including foamed fracturingfluids), gravel pack fluids, viscosifiers used as diverters in acidizing(including foam diverters), VES viscosifiers used to clean up drillingmud filter cake, remedial clean-up of fluids after a VES treatment(post-VES treatment) in regular or foamed fluid forms (i.e. the fluidsmay be “energized”) with or the gas phase of foam being N₂ or CO₂, andthe like.

A value of the methods and compositions herein is that a fracturing orother fluid can be designed to have enhanced breaking characteristics.That is, fluid breaking is no longer dependant on external reservoirconditions for viscosity break and is more controllable: the rate ofviscosity reduction, if complete break is achieved/occurs throughout thereservoir interval. Importantly, better clean-up of the VES fluid fromthe fracture and wellbore may be achieved thereby. Better clean-up ofthe VES directly influences the success of the fracture treatment, whichis an enhancement of the well's hydrocarbon productivity. VES fluidclean-up limitations and limitations of the past may be overcome orimproved by the use of DIAMOND FRAQ™ improved VES gel clean-uptechnology and may now be improved by the use of both types of internalbreakers simultaneously.

In order to practice the methods and compositions herein, an aqueousfracturing fluid, as a non-limiting example, is first prepared byblending a VES into an aqueous fluid. The aqueous fluid could be, forexample, water, brine, aqueous-based foams or water-alcohol mixtures.Any suitable mixing apparatus may be used for this procedure. In thecase of batch mixing, the VES and the aqueous fluid are blended for aperiod of time sufficient to form a gelled or viscosified solution. Asnoted, the internal breakers herein may be added separately or togetherafter the fluid is substantially gelled, in one non-limiting embodiment.In another non-restricted version, a portion or all of the breakingcomposition may be added prior to or simultaneously with the VES gellingagent, in one non-restrictive embodiment, particularly if the breakingagent is in encapsulation form.

Propping agents are typically added to the base fluid after the additionof the VES if a fracturing fluid is being created. Propping agents mayinclude, but are not limited to, for instance, quartz sand grains, glassand ceramic beads, bauxite grains, walnut shell fragments, aluminumpellets, nylon pellets, and the like. The propping agents may benormally used in concentrations between about 1 to 14 pounds per gallon(120-1700 kg/m³) of fracturing fluid composition, but higher or lowerconcentrations may be used as the fracture design required. The basefluid may also contain other conventional additives common to the wellservice industry such as water wetting surfactants, non-emulsifiers andthe like. As noted herein, the base fluid can also contain othernon-conventional additives which can contribute to the breaking actionof the VES fluid, and which are added for that purpose in onenon-restrictive embodiment.

Any or all of the above mineral oils, hydrogenated polyalphaolefin oils,saturated fatty acids and/or unsaturated fatty acids may be provided inan extended release form such as encapsulation by polymer or otherwise,pelletization with binder compounds, absorbed or some other method oflayering on a microscopic particle or porous substrate, and/or acombination thereof. Specifically, the internal breakers may be micro-and/or macro-encapsulated to permit slow or timed release thereof. Innon-limiting examples, the coating material may slowly dissolve or beremoved by any conventional mechanism, or the coating could have verysmall holes or perforations therein for the internal breakers within todiffuse through slowly. For instance, a mixture of fish gelatin and gumacacia encapsulation coating available from ISP Hallcrest, specificallyCAPTIVATES® liquid encapsulation technology, may be used to encapsulatethe internal breakers herein. Also, polymer encapsulation coatings suchas used in fertilizer technology available from Scotts Company,specifically POLY-S® product coating technology, or polymerencapsulation coating technology from Fritz Industries could possibly beadapted to the methods herein. The internal breakers may also beabsorbed onto zeolites, such as Zeolite A, Zeolite 13X, Zeolite DB-2(available from PQ Corporation, Valley Forge, Pa.) or Zeolites Na-SKS5,Na-SKS6, Na-SKS7, Na-SKS9, Na-SKS10, and Na-SKS13, (available fromHoechst Aktiengesellschaft, now an affiliate of Aventis S.A.), and otherporous solid substrates such as MICROSPONGE™ substrates (available fromAdvanced Polymer Systems, Redwood, Calif.) and cationic exchangematerials such as bentonite clay or placed within microscopic particlessuch as carbon nanotubes or buckminster fullerenes. Further, theinternal breakers may be both absorbed into and onto porous or othersubstrates and then encapsulated or coated, as described above.

In a typical fracturing operation, the fracturing fluid is pumped at arate sufficient to initiate and propagate a fracture in the formationand to place propping agents into the fracture. A typical fracturingtreatment would be conducted by mixing a 20.0 to 60.0 gallon/1000 galwater (volume/volume—the same values may be used with any SI volumeunit, e.g. 60.0 liters/1000 liters) amine oxide VES, such as SURFRAQ™,in a 2 to 7% (w/v) (166 lb to 581 lb/1000 gal, 19.9 kg to 70.0 kg/m³)KCl solution at a pH ranging from about 6.0 to about 9.0. The breakingcomponents are typically added before or during the VES addition usingappropriate mixing and metering equipment, or if needed in a separatestep after the fracturing operation is complete or on the fly when goingdownhole. One unique aspect of the UFA breaking chemistry is how theplant, fish and like type oils may be added and dispersed within thebrine mix water prior to the addition of VES, such as the suction sideof common hydration units or blender tubs pumps. These oils, used at thetypical concentrations needed to achieve quick and complete break, donot initially act as detrimental oils and degrade VES yield and thelike. However, most other oils have a detrimental effect to VES yield ifalready present or when added afterwards. One novelty of the enoic-typeoils described herein is they are VES-friendly initially but over timeand a given temperature, or in the presence of the mineral oil internalbreakers become aggressive VES gel breakers. By “VES-friendly” is meantthey are compatible therewith and do not immediate decrease viscosity ofaqueous fluids gelled with VES as is seen with many other oils.

In one non-limiting embodiment, the methods and compositions herein arepracticed in the absence of gel-forming polymers and/or gels or aqueousfluids having their viscosities enhanced by polymers. However,combination use with polymers and polymer breakers may also be ofutility. For instance, polymers may also be added to the VES fluidsherein for fluid loss control purposes. Types of polymers that may serveas fluid loss control agents are various starches, polyvinyl acetates,polylactic acid, guar and other polysaccharides, gelatins, and the like.

The present invention will be explained in further detail in thefollowing non-limiting Examples that are only designed to additionallyillustrate the invention but not narrow the scope thereof.

General Procedure for Examples

To a blender were added tap water, the indicated proportion of salt,followed by the indicated proportion of viscoelastic surfactant (WG-3LVES-AROMOX® APA-T available from Akzo Nobel). The blender was used tomix the components on a very slow speed, to prevent foaming, for about30 minutes to viscosify the VES fluid. Mixed samples were then placedinto plastic bottles. Various components singly or together, in variousconcentrations, were then added to each sample, and the sample wasshaken vigorously for 60 seconds. The samples were placed in a waterbath at the indicated temperature and visually observed every 30 minutesfor viscosity reduction difference between the samples. Since a goal ofthe research was to find a relatively rapid gel breaking composition,samples were only observed for 24 hours or less.

Viscosity reduction may be visually detected. Shaking the samples andcomparing the elasticity of gel and rate of air bubbles rising out ofthe fluid may be used to estimate the amount of viscosity reductionobserved. Measurements using a Brookfield PVS rheometer at the indicatedtemperatures and pressures at 100 sec⁻¹ were used to acquirequantitative viscosity reduction of each sample.

Formulations 1-5 of FIG. 1

The four formulations which gave the results shown in FIG. 1 had thefollowing compositions:

-   Formulation #1: This is simply the base fluid with no internal    breakers. All of the formulations, including the base fluid,    contained these components: 9% KCl+6% WG-3L VES+20.0 pptg (2.4    kg/m³) FLC-40 fluid loss control agent (slurried MgO powder mixed in    monopropylene glycol, available from Baker Oil Tools).-   Formulation #2: This formulation was the base fluid of Formulation    #1 also including 6.0 gptg GBW-402L internal breaker (ConocoPhillips    Pure Performance Base Oil 225N type mineral oil—a fairly high    molecular weight mineral oil).-   Formulation #3: This formulation was the base fluid of Formulation    #1 also including 6 gptg GBW-407L internal breaker (Fish Oil 18:12    TG high EPA and DHA unsaturated fatty acid oil product from    Bioriginal Food & Science Corporation).-   Formulation #4: This formulation was the same as Formulation #3    except that it also contained 0.3 gptg GBC-4L transition metal ion    rate controller (15% bw CuCl₂.2H₂O solution).-   Formulation #5: This formulation was the base fluid of Formulation    #2 also including 6 gptg GBW-407L internal breaker and 0.3 gptg    GBC-4L transition metal ion rate controller.

FIG. 1 presents curves for each Formulation showing the viscositybreaking results as a function of time. The fluid heat-up for each fluidto 134° F. (57° C.) was approximately 20 minutes. Formulation 1, whichwas simply the base fluid with no internal breaker, predictably had themost stable viscosity. The addition of the mineral oil internal breakeronly (Formulation #2) was also very stable at these conditions. The useof the polyenoic acid breaker in Formulation #3 did reduce the viscositymore than the mineral oil of Formulation #2, but it was still relativelystable after leveling off. The addition of a rate controller toFormulation #3 gave Formulation #4; the inclusion of the rate controllertogether with the polyenoic acid internal breaker gave a noticeable dropin viscosity which leveled off and dropped slowly after about threehours. The addition of a mineral oil internal breaker to Formulation #4gave Formulation #5 which surprisingly produced a fairly immediate dropin viscosity that was almost completely broken after 36 hours. Theseformulations showed that the use of a mineral oil internal breakertogether with a polyenoic acid internal breaker unexpectedly buteffectively accelerated breaking in a high salinity composition mixwater.

Formulations of FIG. 2

The aqueous base fluid used in the experiments of FIG. 2 had 14.0 ppg(1.7 kg/liter) CaBr₂, 5.0% WG-3L VES, and 15 pptg (1.8 kg/m³) FLC-40fluid loss control agent. The FLC-40 agent was added to the mix waterbefore the WG-3L VES. For these Formulations the fluid heat-up to 221°F. (105° C.) was approximately 20 minutes.

In Formulation #6, 5 gptg GBW-407L polyenoic acid internal breaker wasadded to the mix water after the WG-3L VES. This formulation gave anoticeable break very quickly, but at about four hours the viscositybegan to recover and reached a peak of about 200 100 sec⁻¹ at about 11hours, after which the viscosity gradually declined. Formulation #7 didnot contain any polyenoic acid internal breaker, but did contain 10.0gptg GBW-402L mineral oil internal breaker. It immediately gave aviscosity of about 230 100 sec⁻¹ which did not decrease over the 16 hourtest period. However, in Formulation #8, 3.0 gptg GBW-407L polyenoicacid internal breaker (less than that used in Formulation #6) and 3.0gptg GBW-402L mineral oil internal breaker (less than one-third of thatused in Formulation #7) were used and surprisingly the viscosity beganto break after about one hour and was essentially completely brokenafter about 7 hours. It may be noticed that the use of both the mineraloil internal breaker and the polyenoic acid internal breaker gave anunexpected synergistic result much more rapidly and completely thaneither of them had given separately at individually greater amounts.

As may be seen, the method of gel breaking described herein is simple,effective, safe, and highly cost-effective. A method is provided forenhancing the breaking the viscosity of aqueous treatment fluids gelledwith viscoelastic surfactants (VESs). Compositions and methods are alsofurnished herein for breaking VES-surfactant fluids controllably,completely and relatively quickly with the use of both types of internalbreakers described herein.

Compositions and methods are also disclosed herein for enhancing thebreaking of VES-surfactant fluids where contact with reservoir fluids'external breaking mechanism is not required, although in someembodiments heat from the reservoir may help the breaking process.Compositions and methods are additionally provided for breakingVES-surfactant fluids where the different types of breaking additivesare in a phase internal to the VES-surfactant fluid. Further, methodsand VES fluid compositions are described herein for breaking theviscosity of aqueous fluids gelled with viscoelastic surfactants usingreadily available materials at relatively inexpensive concentrations,particularly when the two types of internal breakers are used together.

In one non-limiting explanation, it appears the internal breakers becomedispersed throughout the VES micelles and appear to then bethermodynamically weak points where polyenoic autooxidation productsand/or the mineral oil requires less thermodynamic energy to disturb theouter layer of “associated hydrophillic head groups and counterions” (K,Ca, etc. counterions) to degrade VES micelle viscosity by micellerearrangement. This method to degrade VES micelles is not seen asspontaneous or very abrupt but rather a gradual mechanism for a givenfluid temperature, VES loading, mix water salinity, type and amount ofinternal breakers, and the like, that can be crafted to allow controlviscosity break over time, and can give an enhanced viscosity breakingcompared to the cases where polyenoic or mineral oil breakers alone areused to access, disassociate, and/or disturb the VES micelles and reducethe fluid viscosity.

In the foregoing specification, the invention has been described withreference to specific embodiments thereof, and has been demonstrated aseffective in providing methods and compositions for a VES fracturingfluid breaker mechanism. However, it will be evident that variousmodifications and changes can be made thereto without departing from thebroader spirit or scope of the invention as set forth in the appendedclaims. Accordingly, the specification is to be regarded in anillustrative rather than a restrictive sense. For example, specificcombinations of viscoelastic surfactants, mineral oil internal breakers,polyenoic acid internal breakers, and other components falling withinthe claimed parameters, but not specifically identified or tried in aparticular composition or fluid, are anticipated to be within the scopeof this invention.

The word “comprising” as used throughout the claims is to be interpretedto mean “including but not limited to.”

1. An aqueous fluid comprising: at least one viscoelastic surfactant(VES) in an amount effective to increase the viscosity of the aqueousfluid; at least one first internal breaker in an amount effective toreduce the viscosity of the gelled aqueous fluid, where the firstinternal breaker is a mineral oil; and at least one second internalbreaker in an amount effective to reduce the viscosity of the gelledaqueous fluid, where the second internal breaker is selected from thegroup consisting of polyenoic acids, monoenoic acids and combinationsthereof.
 2. The aqueous fluid of claim 1 where the first internalbreaker is present in an amount from about 50 to about 25,000 ppm basedon the total fluid; and when the second internal breaker is present inan amount from about 100 to about 20,000 ppm based on the total fluid.3. The aqueous fluid of claim 1 where the first internal breaker is atleast about 99 wt % paraffin.
 4. The aqueous fluid of claim 1 where thefirst internal breaker has a distillation temperature in the range fromabout 160 to about 550° C., and a kinematic viscosity at 40° C. of fromabout 1 to about 250 cSt.
 5. The aqueous fluid of claim 1 where thesecond internal breaker is a polyenoic acid selected from the groupconsisting of linoleic acid, omega-3 fatty acids, omega-6 fatty acids,stearidonic acid, eleostearic acid, eicosadienoic acid, eicosatrienoicacid, arachidonic acid or eicosatetraenoic acid, eicosapentaenoic acid,docosapentaenoic acid, docosahexaenoic acid, cis-linoleic acid,cis-linolenic acid, gamma-linolenic acid, and conjugated polyenes. 6.The aqueous fluid of claim 1 where the second internal breaker is amonoenoic acid selected from the group consisting of obtusilic acid,caproleic acid, lauroleic acid, linderic acid, myristoleic acid,physeteric acid, tsuzuic acid, palmitoleic acid, petroselinic acid,oleic acid, vaccenic acid, gadoleic acid, gondoic acid, cetoleic acid,nervonic acid, erucic acid, elaidic acid, and t-vaccenic acid.
 7. Theaqueous fluid of claim 1 where the internal breakers are present in anoil-soluble internal phase of the aqueous fluid.
 8. The aqueous fluid ofclaim 1 where the viscosity of the fluid breaks more rapidly with boththe first and second internal breakers as compared with an identicalaqueous fluid containing the same amount of either the first internalbreaker or the second internal breaker alone.
 9. An aqueous fluidcomprising: at least one viscoelastic surfactant (VES) in an amounteffective to increase the viscosity thereof; at least one first internalbreaker in an amount from about 50 to about 25,000 ppm based on thetotal fluid, where the first internal breaker is a mineral oil; and atleast one second internal breaker in an amount from about 100 to about20,000 ppm based on the total fluid, where the second internal breakeris a polyenoic acid, where the viscosity of the fluid breaks morerapidly with both the first and second internal breakers as comparedwith an identical aqueous fluid containing the same amount of either thefirst internal breaker or the second internal breaker alone.
 10. Theaqueous fluid of claim 9 where the second internal breaker is selectedfrom the group consisting of linoleic acid, omega-3 fatty acids, omega-6fatty acids, stearidonic acid, eleostearic acid, eicosadienoic acid,eicosatrienoic acid, arachidonic acid or eicosatetraenoic acid,eicosapentaenoic acid, docosapentaenoic acid, docosahexaenoic acid,cis-linoleic acid, cis-linolenic acid, gamma-linolenic acid, andconjugated polyenes.
 11. The aqueous fluid of claim 9 where the internalbreakers are present in an oil-soluble internal phase of the aqueousfluid.
 12. An aqueous fluid comprising: at least one viscoelasticsurfactant (VES) in an amount effective to increase the viscosity of theaqueous fluid; at least one first internal breaker in an amounteffective to reduce the viscosity of the gelled aqueous fluid, where thefirst internal breaker is a mineral oil; and at least one secondinternal breaker in an amount effective to reduce the viscosity of thegelled aqueous fluid, where the second internal breaker is selected fromthe group consisting of: polyenoic acids selected from the groupconsisting of linoleic acid, omega-3 fatty acids, omega-6 fatty acids,stearidonic acid, eleostearic acid, eicosadienoic acid, eicosatrienoicacid, arachidonic acid or eicosatetraenoic acid, eicosapentaenoic acid,docosapentaenoic acid, docosahexaenoic acid, cis-linoleic acid,cis-linolenic acid, gamma-linolenic acid, and conjugated polyenes,monoenoic acids selected from the group consisting of obtusilic acid,caproleic acid, lauroleic acid, linderic acid, myristoleic acid,physeteric acid, tsuzuic acid, palmitoleic acid, petroselinic acid,oleic acid, vaccenic acid, gadoleic acid, gondoic acid, cetoleic acid,nervonic acid, erucic acid, elaidic acid, and t-vaccenic acid, andcombinations thereof; where the internal breakers are present in anoil-soluble internal phase of the aqueous fluid.
 13. The aqueous fluidof claim 12 where the first internal breaker is present in an amountfrom about 50 to about 25,000 ppm based on the total fluid; and when thesecond internal breaker is present in an amount from about 100 to about20,000 ppm based on the total fluid.
 14. The aqueous fluid of claim 12where the first internal breaker is at least about 99 wt % paraffin. 15.The aqueous fluid of claim 12 where the first internal breaker has adistillation temperature in the range from about 160 to about 550° C.,and a kinematic viscosity at 40° C. of from about 1 to about 250 cSt.16. The aqueous fluid of claim 12 where the viscosity of the fluidbreaks more rapidly with both the first and second internal breakers ascompared with an identical aqueous fluid containing the same amount ofeither the first internal breaker or the second internal breaker alone.